Wireless local area network apparatus

ABSTRACT

A wireless local area network apparatus includes a transmitter and a receiver in which operation of the receiver is accurately synchronized with periodic signals from the transmitter. The periodic signals contain timing data indicating the state of a timer in the transmitter at the time the signal containing that data was transmitted and this timing data is retrieved from the signal when received by the receiver and loaded in a timer for controlling operation of the receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 11/331,919, filed on Jan.13, 2006 as attorney docket no. Diepstraten 33-8-7, which is acontinuation of application Ser. No. 10/681,267, filed on Oct. 9, 2003as attorney docket no. Diepstraten 29-7-6 and issued on Mar. 7, 2006 andU.S. Pat. No. 7,010,058, which is a divisional of application Ser. No.10/092,295, filed on Mar. 7, 2002 as attorney docket no. Diepstraten24-6-5 and issued on Mar. 16, 2004 as U.S. Pat. No. 6,707,867, which isa continuation of Ser. No. 08/155,661, filed on Nov. 22, 1993 asattorney docket no. Diepstraten 11-3-4 and now abandoned, which claimedforeign priority from British patent application 9304622.5, filed onMar. 6, 1993.

The subject matter of this application is related to U.S. patentapplication Ser. No. 11/726,840 filed Mar. 23, 2007 as attorney docketno. Diepstraten 34-9-8, the teachings of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless local area network apparatus.

2. Description of the Related Art

A wireless local area network commonly comprises a plurality ofcommunication stations located in a Basic Service Area (BSA). Thestations can send and receive communication signals via a base stationand, in this manner, the base station receives the signals from astation in the BSA and re-transmits the signals to the intendedrecipient station.

The BSA can be provided as one of a plurality of BSAs which togetherform an Extended Service Area. In this case, the base station of eachBSA may comprise an access point for a backbone infrastructure forconnecting the BSAs for allowing communication between stations indifferent BSAs within the Extended Service Area.

Communication between stations, whether by way of a base station orotherwise, can require synchronization between a transmitter of onestation or an access point and a receiver of another station.Disadvantageously, accurate synchronization between a transmitter and areceiver in a BSA cannot be readily achieved due, in particular, tooperational limitations such as transmission and reception delays anddelays in accessing the wireless medium.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide wireless local areanetwork apparatus having improved synchronization between thetransmitters and the receivers in the network.

According to certain embodiments of the present invention, there isprovided wireless local area network apparatus comprising transmittermeans and receiver means, characterized in that said transmitter meansincludes transmitter timer means for controlling periodic generation oftransmission signals, said receiver means includes receiver timer means,and said transmitter means has means for including transmitter timerdata in said signals for synchronizing said receiver timer means withsaid transmitter timer means, said transmitter timer data representingthe state of said transmitter timer means at the time of transmission ofthe signal in which it is included.

The wireless local area network apparatus of certain embodiments of thepresent invention is particularly advantageous for power managementapplications in which low power portable wireless stations are employedin the BSA. The stations periodically switch between a low powerconsumption state, in which their transceivers are de-energized, and ahigh power consumption state, in which their transceivers are energized,and can thereby receive periodic signals transmitted from some otherstation. The synchronization between the signals transmitted from someother station and the switching of the power-consumption state of thereceiver stations is advantageously achieved by the apparatus of thepresent invention. The improved synchronization of the present inventionallows for operation of the stations in a wireless local area networkwith reduced power-consumption, which is particularly important forstations having an on-board power supply.

The apparatus of certain embodiments of the present invention can beadvantageously employed to control other timing relationships between atransmitter and a receiver in a wireless local area network. Forexample, in so-called frequency-hopping devices, the transmissionfrequency employed by a transmitter is periodically changed and so areceiver has to adapt to this change in communication-signal frequency.The apparatus of certain embodiments of the present invention allows foraccurate synchronization between the operational changes in thetransmitter and receiver during such frequency hopping.

In one embodiment, the present invention is a receiver for a wirelesslocal area network (WLAN). The receiver comprises a radio modem adaptedto receive, from a transmitter of the WLAN, a transmission signalcontaining a time stamp value; a first register adapted to receive thetransmission signal from which the time stamp value is retrieved; atimer adapted to initiate a count sequence based on the time stamp valueand generate a timer control signal at the completion of the countsequence; and a controller adapted to control operations of the receiverbased on the timer control signal from the timer.

In another embodiment, the present invention is a method for a receiverin a WLAN. A transmission signal containing a time stamp value isreceived from a transmitter of the WLAN. The time stamp value isretrieved from the transmission signal, and a count sequence isinitiated based on the time stamp value. A timer control signal isgenerated at the completion of the count sequence, and operations of thereceiver are controlled based on the timer control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention is described further hereinafter, withreference to the accompanying drawings in which:

FIG. 1 shows a wireless local area network which forms part of anextended service area;

FIG. 2 is a block diagram of a transmitter for use in apparatusembodying the present invention;

FIG. 3 shows the structure of a Traffic Indication Message constructedin the transmitter of FIG. 2;

FIG. 4 is a flow diagram of the operation of the transmitter of FIG. 2;

FIG. 5 is a block diagram of a receiver for use in apparatus embodyingthe present invention;

FIG. 6 is a flow diagram of the operation of the receiver of FIG. 5; and

FIG. 7 is a timing diagram illustrating operation of the transmitter ofFIG. 2 and the receiver of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

As mentioned above, the apparatus of the present invention can be usedin a power management system for a wireless local area network.

Such a local area network is shown in FIG. 1 and comprises a basicservice area (BSA) 10 having six mobile stations 12.1-12.6 locatedtherein. In the illustrated embodiment each of the stations 12.1-12.6 ispowered by an on-board d.c. supply (not shown) although some of thestations could be supplied by connection to an a.c. source. An accesspoint 14 is also located in the BSA 10 and is typically connected to ana.c. power supply (not shown) and is connected to a backbone structure18 linking the access point 14 to access points of other BSAs (notshown). The stations 12.1-12.6 communicate with each other via theaccess point 14. Thus, a communication signal from one station 12.1 toanother station 12.2 will not be received directly by the station 12.2but will first be received by the access point 14 and then transmittedto the station 12.2.

In order to reduce the power consumption of the stations 12.1-12.6, andthereby increase the operational life-time before the on-board d.c.power supply needs to be recharged or replaced, the stations 12.1-12.6are operated in a power-save-mode in which their transceivers areperiodically de-energized and the station is then in a so-called dozestate. In order to operate the station 12.1-12.6 in a power-save-modewithout losing any transmitted data packets, a data packet that isintended for a station that is in a doze state is buffered in the accesspoint 14 until such time as the station wakes-up from its doze stateinto a so-called awake state and energizes its transceiver to receivethe buffered data.

Traffic Indication Message (TIM) packets are transmitted at regularintervals from the access point 14 and indicate for which stations12.1-12.6 in the BSA 10 data packets are buffered in the access point14. The transceivers in the stations 12.1-12.6 are periodicallyenergized at regular intervals such that the stations 12.1-12.6 wake upfrom a doze state to receive the TIM packets transmitted by the accesspoint 14. If a TIM packet received indicates that a data packet isbuffered in the access point 14 for one of the stations 12.1-12.6, thetransceiver of that station either waits to receive the data packetwhich is arranged to automatically follow the TIM packet, or the stationtransmits a poll packet to the access point 14 to request that the datapacket be transmitted. In both of the above situations, the transceiverin the station remains in an energized state once it has received a TIMpacket indicating that data is buffered for that station. Once the datapacket has been received, the station returns to a doze state until itawakes to receive another TIM packet.

Accordingly, with the exception of the periodic waking to receive theTIM packets, a station 12.1-12.6 remains in a power saving doze stateunless a TIM packet indicates a data packet is buffered for thatstation. In this manner, the power consumption of each station 12.1-12.6is reduced and the operational life-time, i.e. the time beforerecharging or replacement of the d.c. power source is necessary, of thestation is increased. The improved synchronization provided by thepresent invention provides for improved synchronization between theaccess point 14 and the stations 12.1-12.6 operating in a power-savemode so as to achieve advantageously reduced power consumption in thestations 12.1-12.6.

Further power consumption reductions can be achieved by operation of thestations 12.1-12.6 in a so-called extended-power-save mode. The improvedsynchronization provided by the present invention advantageouslysupports operation of the stations 12.1-12.6 in the extended-power-savemode. In this mode, the station is controlled to wake up from a dozestate to receive only every xth TIM packet transmitted by the accesspoint 14. For example, if x=150 then the station awakes to receive onlyevery 150th TIM packet transmitted by the access point 14 and so thestation remains in a doze state for a longer period than if it wakes toreceive every TIM packet transmitted by the access point 14. Powerconsumption in the station is thereby further reduced. Since, in theabove example, a station awakes only every 150 TIM packets, accuratesynchronization between the access point 14 and the station is requiredso that the station wakes up at an appropriate time to receive every150th TIM packet. The present invention provides for such accuratesynchronization.

It should be noted that although the access point 14 may have a datapacket buffered therein to transmit to a station operating in anextended-power-save mode, the data packet remains buffered in the accesspoint 14 until the station 12 wakes up upon receipt of the xth TIMpacket after which the station will poll the access point 14 to transmitthe buffered packet and so data is not lost.

The energization of the transceivers in the stations 12.1-12.6 and inthe access point 14 can be controlled by timers which include crystaloscillators. Synchronization between the timers in the stations12.1-12.6 and the access point 14 is achieved by apparatus embodying thepresent invention and an indication of the reduced power consumption ofa station having such a timer and operating in an extended-power-savemode is given below in which:

The time interval between successive TIM packets transmitted from theaccess point 14 is 200 msec; the station's transceiver has a power-updelay of 1 msec; the timing drift of the oscillator in the station is100 micro sec/sec; the timing drift of the oscillator in the accesspoint 14 is 100 micro sec/sec; the TIM packet medium access delay isbetween 0 and 5 msec; and the station is required to wake up to receiveevery 150th TIM packet from the access point 14.

Using the above values as examples:

-   -   The station doze interval=150×200 msec=30 sec    -   The maximum drive of each oscillator in the doze interval=100        micro sec/secx30=3 msec    -   The maximum drift for both oscillators therefore =6 msec

Thus, in view of the station's 1 msec power-up delay, the station shouldwake up 7 msec before the expected TIM packet to compensate for theoscillator drift and the power-up delay.

With a TIM access delay of 5 msec as an example, the period during whichthe station is in an awake state to receive a TIM packet is between 1msec (when there is no crystal drift and the TIM access delay is 0 msec)and 1 msec+6 msec+5 msec=12 msec (when the total crystal drift isexperienced and the TIM interval delay is 5 msec).

Assuming that the TIM packet has a duration of 0.5 msec, the averageduration of the awake state of the station is 1+6/2+5/2+0.5=7 msec.

Thus, in this example, the station will be in an awake state, i.e., withits transceiver energized, for, on average, only 7 msec every 30 secwhich provides for a particularly advantageous power consumptionreduction.

By way of comparison, and assuming the same values as above, if thestation wakes-up at every TIM, thereby requiring an average “on-time” of1+5/2=3.5 msec per 200 msec TIM interval, the station is then awake for525 msec every 30 sec.

FIG. 2 illustrates a transmitter 20 for use in the access point 14. Thetransmitter 20 includes a modulo n counter 22 which, in operation, isfree running and synchronized with a similar modulo n counter 58 in astation's receiver (see FIG. 5).

The modulo n counter 22 functions as a timer and when the count valuereaches n, a TIM function generator 24 is triggered by way of aninterrupt signal 25 indicating that the next TIM packet should beconstructed, and transmitted by way of a radio modem 26.

The TIM packet 28 is constructed in a transmitter buffer 30 and anexample of a TIM packet is illustrated in FIG. 3. The TIM packetcomprises a wireless medium access (WMAC) header and a data fieldformat. The WMAC header includes, amongst other fields, a Type fieldthat identifies the packet as a TIM packet.

The data field format includes:

-   -   A TIME STAMP FIELD in which is loaded a so-called time stamp of        the value of the modulo n counter in the transmitter 20 at the        time of transmission of the TIM;    -   A TIMER INTERVAL FIELD which indicates the value of n of the        modulo n counter in the transmitter 20;    -   A TRAFFIC PENDING FIELD which indicates for which stations data        packets are buffered; and    -   A TRAFFIC BROADCAST PENDING FIELD which indicates the number of        outstanding broadcast data packets buffered for the stations.

Referring again to FIG. 2, once the TIM packet 28 has been constructed,it is delivered to a multiplexer 32 where the time stamp, and cyclicredundancy check (CRC) data from a CRC generator 34, are loaded into theTIM packet 28. A WMAC control 36 controls access to the medium via themodem 26 so that the TIM packet 28 is not transmitted from the accesspoint 14 immediately upon generation of the interrupt signal 25. TheWMAC control 36 follows a medium access protocol such as Carrier SenseMultiple Access with Collision Avoidance (CSMA/CA). According to theCSMA/CA protocol, the energy level on the wireless medium is sensed bythe modem 26 to determine if there is any existing network activity, andif the sensed energy level is above a threshold value, a medium busysignal 40 is delivered from the modem 26 to the WMAC Control 36. If nomedium busy is issued, so the medium is sensed “free,” the WMAC control36 turns on the transmitter of the modem 26 by issuing a request to send(RTS) signal. The modem 26 will then start to send a training sequenceand will issue a clear-to-send signal (CTS) once the training sequenceis complete. The modem 26 then sends the serialized data that arrivesfrom the buffer via the multiplexer 32 and a shift register 44. If themedium is sensed as “busy,” the WMAC control 36 waits until the mediumbecomes free and then generates a random backoff delay after which themedium is again sensed. If the medium is sensed as “free” at this pointthen the control 36 follows the RTS, CTS procedure above.

When accessing the medium and once the training sequence has ended, themodem 26 provides the CTS 42 and the TIM packet stored in the buffer 30is loaded into the shift register 44 via the multiplexer 32. Oncetransmission of the header has started, the time stamp is loaded fromthe timer 22 into the shift register 44 via the multiplexer 32 and underthe control of a transmit control circuit 43 in the WMAC control 36. Thetransmit control circuit 43 also controls the start of the transmissionof the header. As mentioned above, the modulo n counter 22 in the accesspoint 14 of transmitter 20 is free running and so by the time theCSMA/CA protocol has been completed, and particularly if a medium busysignal 40 was received by the WMAC control 36, the counter 22 is alreadyinto its next count sequence, i.e. at a value between 0 and n, by thetime that the clear-to-send signal 42 is received by the WMAC control36. At a predetermined time relative to the clear-to-send signal 42,which predetermined time is an accurate estimation of the exact time atwhich the TIM packet will be transmitted having regard to delays in themodem 26, the so-called “time stamp” i.e. the value of the modulo ncounter 22 at that predetermined time, will be loaded in the TIM packet28 stored in the buffer 30. The TIM packet 28 is loaded into a shiftregister 44 upon generation of a load signal 46 from the WMAC control36, and then transmitted by way of the modem 26.

FIG. 4 further illustrates the operation of the transmitter 20 outlinedabove.

FIG. 5 illustrates a receiver 48 of one of the stations 12.1-12.6 in theBSA which is arranged to receive a TIM packet 28 and a data packet (notshown) from the access point 14.

The operation of the receiver 48 is outlined below and furtherillustrated in FIG. 6.

Energization of the receiver 48 is controlled by a modulo n counter 58which functions as a timer to wake up the station 12.1 from a doze stateto receive the TIM packet 28 transmitted from the access point 14.

The TIM packet 28 is received by a receiver modem 50 and its time stampvalue retrieved from the TIM TIME STAMP FIELD (FIG. 3). The retrievedtime stamp is delivered by way of a shift register 52 to a counterregister 54 which commences a modulo n count starting from the pointbetween 0 and n which corresponds to the time stamp value. The counterregister 54 continues its modulo n count with the same clock signal 56that controls the modulo n counter 58. This modulo n count is stored inthe counter register 54 until the TIM packet 28 is completely receivedand the CRC data checked. If the CRC is correct, the modulo n count isloaded from the counter register 54 into the modulo n counter 58. Theuse of the counter register 54 is particularly advantageous in that itallows TIM packets of different lengths to be received. This arisessince the modulo n count sequence, that commences at the time stampvalue, is buffered in the register 54 while the TIM packet 28 isprocessed completely. The counter register 54 maintains the cyclicmodulo n count for as long as is necessary to process the TIM packet.

If all the TIM packets are of the same known length, then aTIM-packet-processing compensation factor could be applied to the timestamp value to allow for the known time taken to process the TIM packetof known length. The compensated time stamp value would then be loadeddirectly into the modulo n counter 58 and so the intermediate counterregister 54 would not be required.

Referring again to the embodiment illustrated in FIG. 5, a delaycompensation value 60 is added to the modulo n count by an adder 62 asthe count is transferred from the counter register 54 to the modulo ncounter 58. The compensation value 60 compensates for the propagationdelay of the receiver 48 and the transmitter 20. Once the compensatedmodulo n counter value is transferred from the counter register 54 tothe counter 58, the counter 58 is then accurately synchronized with themodulo n counter 22 in the transmitter (FIG. 2).

Once the modulo n counters 22, 58 in the station 12.1 and the accesspoint 14 are accurately synchronized, the counter 58 provides thestation 12.1 with an accurate indication of the time at which thecounter 22 in the access point 14 reaches its n value and generates aTIM packet for transmission. Since the counter 22 in the access point 14remains free-running, and the counter 58 in the station 12.1 isaccurately synchronized with the counter 22, the station 12.1 can becontrolled to accurately wake up in time to receive only every xth TIMpacket without requiring the station 12.1 to wake up unnecessarily earlyas would be required to assure receipt of the TIM packet if accuratesynchronization between the counters 22, 58 was not available. Thereduction in the need for early wake up of the station 12.1advantageously reduces the power consumption of the station 12.1.

It should be noted that each station 12.1-12.6 in the BSA 10 can operatewith different doze intervals. For example one of the stations 12.1 canbe controlled to wake up every 150 TIM packets while another station12.2 wakes up every 200 TIM packets. Each time the station 12.1 wakes upto receive a TIM packet, the modulo n counter 58 is reset by the timestamp retrieved from the TIM packet so that continued accuratesynchronization can be achieved.

FIG. 7 is a timing diagram that further illustrates the improvedsynchronization of the present invention as provided in a powermanagement application. The access point 14 activity indicates thetransmission of the first five TIM packets 64-72, and the last TIMpacket 73, of a one hundred and fifty TIM packet series and the firstfive TIM generation signals 74-82 generated each time the modulo ncounter 22 in the access point 14 reaches its value n. As shown, thetransmission of the first TIM packet 64 is delayed due to a medium busysignal obtained from the CSMA/CA protocol. The first TIM packet 64 istherefore actually transmitted m counts of modulo n counter 22 into thefirst count sequence 74-76. The station 12.1 has previously beensynchronized to wake up at 84 to receive the first TIM packet 64. TheTIM packet 64 carries a time stamp value m representing the value of themodulo n counter 22 in the access point 14 at the actual time oftransmission of the TIM packet 64. As described above, the station 12.1retrieves the time stamp from the TIM packet 64 and loads it into itsown modulo n counter 58 which then commences its count sequence at valuem. As shown in FIG. 7, the two modulo n counters 22, 58 remain insyncnronization as they cyclically count up to value n. Thissynchronization readily allows the station 12.1 to remain in a dozestate until its modulo n counter 58 indicates that the 150th TIM packet73 is to be generated in, and transmitted from, the access point 14, andthe station 12.1 wakes up at 85. Only a minor amount of compensation isnecessary to allow for the possible modem delay of the transmitter 20and receiver 48.

If a time stamp value of the access point counter 22 is not taken andinstead the station counter 58 is reset to 0 by the actual receipt ofthe TIM packet 64, the late arrival of the TIM packet 64 due to theCSMA/CA delay leads to unsynchronized operation of the counters 22, 58because when the access point counter 22 has reached a value m, thestation counter 58 is being reset to 0 by receipt of the TIM PACKET 64.The station counter 58 has therefore just recorded a TIM interval of n+mcounts and if the station is then controlled to remain in a doze stateuntil 150 TIM packets have been transmitted, i.e. until after 150 TIMintervals, the station erroneously dozes for 150×(m+n) intervals insteadof 150×n intervals and further power consuming compensatory steps arenecessary which disadvantageously reduces the power saved by energizingthe station receiver only every 150 TIM packets.

Thus, by including a time stamp representing the state of the accesspoint counter 22 at the exact time of transmission of the TIM packet,the power saving benefit of energizing the station only every 150 TIMpackets can be increased.

The above describes a preferred embodiment of the integration of thesynchronization function in the medium-access-control function. Otherforms, in which the reference point in time, where the “time stamp” issampled, is available to both the transmitter and the receiver, canutilize the start of the frame or the actual location of the time stampfield.

The invention is not restricted to the details of the foregoingpower-management embodiment. For example, the apparatus of the presentinvention can be employed to provide synchronization of frequencychannel selection in frequency-hopping devices. In such devices the basestation, for example the access point, switches communication operatingfrequency at a precise moment, and it is required that the otherstations in the network are synchronized so as to switch their operatingfrequency to the new frequency at that moment. In accordance with afurther advantage provided by the invention, the access point does notneed to transmit a separate frequency-hop signal each time thecommunication operating frequency is required to change but can includea timing signal for two or more successive frequency-hops which cantherefore be delivered to the stations at intervals that are longer thanthe intervals between the required frequency-hops. Accordingly, thestations can operate in an extended-sleep-mode wherein each xth TIMpacket that is received also includes timing information indicating whenthe station should switch its communication operating frequency. Thus,providing frequency change logic (86 in FIG. 5) remains operationalduring the extended sleep period, the required frequency hop, or hops,can occur during the sleep period so that when the station next wakesup, it is still operating with the same communication frequency as theaccess point. Advantageously, the synchronized timing control of afrequency hopping device can be combined with the power managementfunction of such a device so that the frequency-change logic 86 and astation wake-up control 88 are controlled by the same timing source 58.

1. A receiver for a wireless local area network (WLAN), the receivercomprising: a radio modem adapted to receive, from a transmitter of theWLAN, a transmission signal containing a time stamp value; a firstregister adapted to receive the transmission signal from which the timestamp value is retrieved; a timer adapted to initiate a count sequencebased on the time stamp value and generate a timer control signal basedon the count sequence; and a controller adapted to control operations ofthe receiver based on the timer control signal from the timer.
 2. Theinvention of claim 1, wherein the controller is adapted to controltiming of energizing of the radio modem based on the timer controlsignal to enable the radio modem to receive a subsequent transmissionsignal from the transmitter.
 3. The invention of claim 2, wherein theenergizing of the radio modem corresponds to a transition from a dozestate to an awake state.
 4. The invention of claim 1, wherein initiatingthe count sequence based on the time stamp value synchronizes the timerwith a corresponding timer in the transmitter.
 5. The invention of claim4, wherein the transmitter timer is a free-running timer.
 6. Theinvention of claim 1, wherein the timer comprises a modulo n counter. 7.The invention of claim 1, wherein the transmission signal is a TIM(Traffic Indication Message) packet.
 8. The invention of claim 1,wherein the timer control signal is generated at the completion of thecount sequence.
 9. The invention of claim 1, wherein the time stampvalue corresponds to a transmitter timer value after transmission fromthe transmitter of a header of the transmission signal has started. 10.The invention of claim 1, wherein the time stamp value represents avalue within a count sequence of a timer in the transmitter.
 11. Theinvention of claim 10, wherein the time stamp value represents the valuewithin the count sequence of the timer in the transmitter at a time oftransmission of the transmission signal from the transmitter.
 12. Amethod for a receiver in a wireless local area network (WLAN), themethod comprising: (a) receiving, from a transmitter of the WLAN, atransmission signal containing a time stamp value; (b) retrieving thetime stamp value from the transmission signal; (c) initiating a countsequence based on the time stamp value; (d) generating a timer controlsignal based on the count sequence; and (e) controlling operations ofthe receiver based on the timer control signal.
 13. The invention ofclaim 12, wherein step (e) comprises controlling timing of energizing ofa radio modem in the receiver based on the timer control signal toenable the radio modem to receive a subsequent transmission signal fromthe transmitter.
 14. The invention of claim 13, wherein the energizingof the radio modem corresponds to a transition from a doze state to anawake state.
 15. The invention of claim 12, wherein initiating the countsequence based on the time stamp value synchronizes the receiver with atimer in the transmitter.
 16. The invention of claim 12, wherein thetimer control signal is generated at the completion of the countsequence.
 17. The invention of claim 12, wherein the time stamp valuecorresponds to a transmitter timer value after transmission from thetransmitter of a header of the transmission signal has started.
 18. Theinvention of claim 12, wherein the time stamp value represents a valuewithin a count sequence of a timer in the transmitter.
 19. The inventionof claim 18, wherein the time stamp value represents the value withinthe count sequence of the timer in the transmitter at a time oftransmission of the transmission signal from the transmitter.